Metallurgical coke has a variety of uses. For example, metallurgical coke can be used for friction materials, for conductive flooring, for foundry coatings, for corrosion materials, for foundry carbon raiser, as a reducing agent, in drilling applications, as ceramic packing media, and for heat-treatment, for oxygen exclusion and electrolytic processes. Metallurgical coke can be also used as a filler coke for poly-granular carbon products.
However, the majority of metallurgical coke is used as a fuel, as well as a reducing agent, for producing iron and steel. More specifically, metallurgical coke is added to iron ore in a blast furnace to produce pig iron which can be processed into other products.
In order to make metallurgical coke, coal is heated in the absence of air, and the coal will soften, plasticize, and then re-solidify into coke. More specifically, coal is fed into coke ovens and subjected to oxygen-free pyrolysis, and heated to approximately 1100° C. This melts the coal and drives off any volatile compounds and impurities to leave pure carbon. The purified, hot, liquefied carbon solidifies into lumps called coke that, for example, can be fed into a blast furnace along with iron ore and limestone to produce pig iron that is further processed to produce steel and steel alloys.
In order to be acceptable as metallurgical coke, the resultant coke must have sufficient stability to support the temperatures, and physical and chemical processes in the blast furnace. Thus, the characteristics and qualities of the coal used to make the coke are important in determining if the coal can be used to make metallurgical coke.
Coal is typically divided into four categories (or ranks): anthracite; bituminous; sub-bituminous; and, lignite. Each type of coal has different sets of physical properties that are based upon a variety of factors. Three of the more relevant factors that impact the coal's physical properties are the moisture, volatile content, and carbon content. Additionally, the levels of sulfur and ash in coal are typically used in determining if the coal is good coking coal. For example, a good coking coal may have a moisture content between 2-15%, a volatile content between 32-36%, a carbon content between 45-85%, and ash content between 3-12% and a sulfur content between 0.7-4%.
Additionally, the Crucible Swelling Number (CSN) (also referred to as Free Swelling Index (FSI)) is one qualitative measurement that can be used to evaluate the properties of the coal. A coal with a low CSN, for example between 0 to 2, is believed to be unsuitable for making metallurgical coke. Additionally, a coal with a high CSN, for example 8 or 9, is also believed to be unsuitable for making metallurgical coke.
If the coal meets the appropriate standards and thus, has acceptable physical characteristics as well as low enough levels of impurities), it is considered “good coking coal” (or “metallurgical coal”). On the other hand however, if the coal does meet one or more of the standards for the good coking coal, it is considered “poor coking coal” (or “non-metallurgical coal”) and is not used by itself to make metallurgical coke.
Since good coking coal is both rare and expensive in comparison to poor coking coal, efforts have been made to utilize poor coking coal to produce metallurgical coke. One known method for using poor coking coal to produce metallurgical coke involves mixing the poor coking coal with good coking coal. The mixture typically must provide an acceptable base to acid ratio. The base to acid ratio for the coal is defined as the amount of the basic oxides (Fe2O3, CaO, MgO, K2O, Na2O) divided by the amount of the acidic oxides (SiO2, Al2O3, TiO2). If the resulting mixture has an acceptable base to acid ratio, it can be used to produce metallurgical coke. However, this method still requires the use of good coking coal, which, again is expensive and rare in comparison to poor coking coal.
Another method of using poor coking coal to produce metallurgical coke involves mixing poor coking coal with one or more binders. The binder(s), together with the poor coking coal, can be heated to make a coke that has the appropriate properties associated with metallurgical coke.
Pitch is a viscoelastic polymer that can be derived from a variety of sources and which has been used as a binder with poor coking coal to make metallurgical coke. Pitch can come from a variety of sources, for example, thermal hydrocracking of petroleum residues, petroleum processing by distillation and solvent de-asphalting, and destructive distillation of coal to name a few. The source of the pitch will impact the pitch components (and the pitch properties).
Thermally hydrocracked pitch is readily abundant and comprises a solid waste product from the various reactions of petroleum residues to produce various desirable products like gasoline, diesel, gas oil and other hydrocarbon materials. Typically, thermally hydrocracked pitch comprises a mixture of coke, spent catalyst, and, some heavy hydrocarbons (i.e., hydrocarbons having 20 or more carbon atoms).
While it has been suggested to use neat (i.e., unaltered) thermally hydrocracked pitch as a binder for making metallurgical coke, thermally hydrocracked pitch contains various chemicals and impurities that are or may be undesirable in metallurgical coke.
It would be desirable to provide a process for making a thermally hydrocracked pitch based binder which can be used with poor coking coal to produce acceptable metallurgical coke.